<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .ipt::placeholder {
      color: red;
    }
    /* 并集选择器 设置的placeholder是无效的 */
    .ipt::-webkit-input-placeholder, 
    .ipt::-moz-placeholder, 
    .ipt:-ms-input-placeholder, 
    .ipt:-moz-placeholder { 
        color: #333; 
    }
  </style>
</head>
<body>
  <input class="ipt" type="text" placeholder="请输入内容">
</body>
</html>